In constant voltage power supply circuits including a constant voltage power supply unit, a ripple voltage is inherently generated in an output voltage of the constant voltage power supply unit due to a variety of causes. A ripple filter is known for removing such ripple voltage as disclosed in Patent Reference No. 1.
FIG. 5 shows a conventional constant voltage power supply circuit 100 using a ripple filter.
The constant voltage power supply circuit 100 comprises a constant voltage power supply unit 101 that generates and outputs a predetermined constant voltage, and a ripple filter 102 connected between an output end A and an output terminal OUTa.
The ripple filter 102 comprises an NPN transistor Qa, a resistor Ra and a capacitor Ca. The NPN transistor Qa has a collector connected to the output end A of the constant voltage power supply unit 101, an emitter connected to the output terminal OUTa, and a base connected to a node between the resistor Ra and the capacitor Ca. Another end of the capacitor Ca is grounded and another end of the resistor Ra is connected to the output end A of the constant voltage power supply unit 101. A time constant of the resistor Ra and the capacitor Ca is adjusted so as to be long enough compared with a frequency of the ripple voltage to be removed.
In operation, when the ripple voltage rises, that is, when an output voltage Va of the constant voltage power supply unit 101 rises, a current flowing through the resistor Ra is increased and the increased current charges the capacitor Ca and raises a voltage across the capacitor Ca. However, since the time constant of the resistor Ra and the capacitor Ca is adjusted so as to be long enough compared with the ripple frequency, the voltage across the capacitor Ca does not change significantly during the period of the ripple voltage rising. As a result, a base voltage of the NPN transistor Qa is stable and therefore a voltage Voa at the output terminal OUTa does not change significantly.
On the other hand, when the ripple voltage falls down, that is, when the output voltage Va of the constant voltage power supply unit 101 is lowered, the current flowing through the resistor Ra is decreased and the capacitor Ca discharges. However, since the voltage across the capacitor does not actually change significantly during the period of the ripple voltage falling, the base voltage of the NPN transistor Qa is stable, and therefore the voltage Voa at the output terminal OUTa does not change significantly.
FIG. 6 is a block diagram of another type of conventional constant voltage power supply circuit. The constant voltage power supply circuit shown in FIG. 6 has a DC/DC converter 105, and a series regulator 106 connected between an output end of the DC/DC converter and an output terminal of the constant voltage power circuit. The DC/DC converter 105 inherently generates a high frequency ripple voltage in its output voltage. The ripple voltage is removed by the series regulator 106.
[Patent reference 1] JPA 5-95628
The constant voltage power supply circuit shown in FIG. 5 generates a large voltage drop between the output end A of the constant voltage power supply unit 101 and the output terminal OUTa. The voltage of the capacitor Ca must be higher than the output voltage Voa by a base-emitter voltage Vbe for the NPN transistor Qa. The capacitor Ca is charged through the resistor Ra and therefore the output voltage Va of the constant voltage power supply unit 101 must be high, and therefore power supply efficiency is degraded.
In order to make the time constant of the resistor Ra and the capacitor Ca large enough, either a resistance value of the resistor Ra or a capacitance of the capacitor Ca must be made large.
If the resistance value of the resistor Ra is made large, a base current of the NPN transistor Qa is reduced. It is not desired to reduce the base current of the NPN transistor Qa, because all the current supplied to the output terminal OUTa and a load (not shown) connected thereto flows through the NPN transistor Qa. Therefore, if the resistance value of the resistor Ra becomes large, the output voltage Va of the constant voltage power supply unit 101 should be larger in order to give enough base current of the NPN transistor Qa. As a result, the voltage drop across the NPN transistor Qa is further increased, and therefore the power supply efficiency is degraded.
On the other hand, if the capacitance of the capacitor Ca is made large, the large size of the capacitance Ca makes it impossible to integrate the capacitor Ca and the capacitor Ca has to be externally attached.
Also in the constant voltage power supply circuit shown in FIG. 6, the series regulator 106 has a voltage drop. When an output current of the series regulator 106 is large, the voltage drop thereof also becomes large, resulting in degradation of the power supply efficiency.